The present invention relates to thermoplastic sheet and film products. More particularly, the invention relates to an apparatus for producing selectively apertured thermoplastic sheet or film.
Perforated and embossed thermoplastic sheet or film have many useful applications. Perforated film is used in gardening and farming to prevent the growth of grass and other weeds while permitting more moisture to be transmitted through the film to the soil beneath. Perforated film is also used for making disposable baby diapers. U.S. Pat. No. 3,814,101 discloses diapers employing perforated thermoplastic film which permits the flow of liquid in the direction of the absorbent material in the diaper but substantially reduces the possibility of flow in the opposite direction. Embossed film without perforations are also used in making baby diapers, and in other applications where it is important that the film have a clothlike appearance or feel.
One of the methods for perforating thermoplastic sheet or film is disclosed in U.S. Pat. No. 3,054,148, issued to Zimmerli, which is hereby incorporated by reference. The Zimmerli patent discloses a stationary drum having a molding element mounted around the outer surface of the drum and being adapted to rotate freely thereon. A vacuum chamber is employed beneath the screen or molding element to create a pressure differential between the respective surfaces of the thermoplastic sheet to cause the plasticized sheet to flow into the perforations provided in the molding element and thereby cause a series of holes to be formed in the sheet.
More recently, various apparati have been constructed to produce a variety of types of perforated plastic film. These are illustrated by U.S. Pat. Nos. 4,155,693, 4,157,237, issued to Raley, and U.S. Pat. No. 4,252,516 issued to Raley and Adams. Although all of these devices are effective for their intended purposes, they have limited capability for producing selectively apertured or perforated film.
In general, to successfully aperture or perforate a film on vacuum process equipment, a minimum vacuum level and a minimum melt temperature must be maintained. In order to prevent operating in a selected area one or both of these conditions must be lowered in that area but only to the extent that holes or perforations are not formed and the pattern retained.
Previously, it has been possible to manufacture an apertured/non-apertured thermoplastic film only with the apertured area and non-apertured area alternating across the web of the film and running continuously in the machine direction. Production of a film of this type requires modification of the stationary seals within the vacuum roll or drum. The modification is accomplished by cutting back the seals to provide a controlled vacuum leak. The latter produces the non-perforated area during production of the film.
Since the screen or molding element rotates with the web it is desirable to produce a screen which will produce a selectively apertured film of any desired configuration.
Elimination of the seal cut back requirement facilitates changing from one type of product to another.